Could be air bubbles in the oil right after the fill. With the oil going in, it foams and causes bubbles. We installed a "Tiger Loop" filter at the boiler that removes all the bubbles. Never had any issues once the new filtration system was installed. Not an expensive fix and takes only an hour to install. Might take a look at that since this issue seems to happen after the oil fills. Google Tiger Loop for more info.....
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